1. Field of the Invention
The present invention relates generally to a broadband wireless access communication system, and in particular, to a system and method for determining a handover at the request of a base station in a broadband wireless access communication system employing Orthogonal Frequency Division Multiplexing (OFDM).
2. Description of the Related Art
In a 4th generation (4G) communication system, active research is being conducted on technology to provide users with services guaranteeing various qualities of service (QoSs) at a data rate of about 100 Mbps. The current 3rd generation (3G) communication system generally supports a data rate of about 384 Kbps in an outdoor channel environment having a relatively poor channel environment, and supports a data rate of a maximum of 2 Mbps even in an indoor channel environment having a relatively good channel environment. A wireless local area network (LAN) system and a wireless metropolitan area network (MAN) system generally support a data rate of 20 Mbps to 50 Mbps. Therefore, in the current 4G communication system, the active research is being carried out on a new communication system securing mobility and QoS for the wireless LAN system and the wireless MAN system supporting a relatively high data rate in order to support high-speed services the 4G communication system aims to provide.
Due to its broad service coverage and high data rate, the wireless MAN system is suitable for high-speed communication services. However, because the mobility of a user or a subscriber station (SS), is not taken into consideration, a handover caused by fast movement of the subscriber station is also not considered in the system.
A communication system proposed in IEEE (Institute of Electrical and Electronics Engineers) 802.16a performs a ranging operation between a subscriber station and a base station (BS), for communication. A configuration of the communication system proposed in the IEEE 802.16a according to the prior art will now be described with reference to FIG. 1.
FIG. 1 is a diagram schematically illustrating a configuration of a broadband wireless access communication system employing Orthogonal Frequency Division Multiplexing (OFDM)/Orthogonal Frequency Division Multiple Access (OFDMA) (hereinafter referred to as “OFDM/OFDMA broadband wireless access communication system”). More specifically, FIG. 1 is a diagram schematically illustrating a configuration of an IEEE 802.16a/IEEE 802.16e communication system.
Before a description of FIG. 1 is given, it should be noted that the wireless MAN system is a broadband wireless access (BWA) communication system, and has broader service coverage and supports a higher data rate compared with the wireless LAN system. The IEEE 802.16a communication system is a communication system employing OFDM and OFDMA for supporting a broadband transmission network to a physical channel of the wireless MAN system. That is, the IEEE 802.16a communication system is an OFDM/OFDMA broadband wireless access communication system. The IEEE 802.16a communication system, applying OFDM/OFDMA to the wireless MAN system, transmits a physical channel signal using a plurality of subcarriers, thereby making it possible to support high-speed data communication. The IEEE 802.16e communication system considers mobility of a subscriber station in addition to characteristics of the IEEE 802.16a communication system. However, no specification has been proposed for the IEEE 802.16e communication system. As a result, the IEEE 802.16a communication system and the IEEE 802.16e communication system are both an OFDM/OFDMA broadband wireless access communication system, and for the convenience of explanation, the description will be made with reference to both the IEEE 802.16a communication system and the IEEE 802.16e communication system.
Referring to FIG. 1, the IEEE 802.16a/IEEE 802.16e communication system has a single-cell configuration, and comprises a base station 100 and a plurality of subscriber stations 110, 120, and 130, which are controlled by the base station 100. Signal exchanges between the base station 100 and the subscriber stations 110, 120 and 130 are achieved using the OFDM/OFDMA technology.
FIG. 2 is a diagram schematically illustrating a downlink frame format for an OFDM/OFDMA broadband wireless access communication system, and in particular, illustrating a downlink frame format for an IEEE 802.16a/IEEE 802.16e communication system. Referring to FIG. 2, the downlink frame includes a preamble field 200, a broadcast control field 210, and a plurality of time division multiplexing (TDM) fields 220 and 230. A synchronization signal or a preamble sequence, for acquiring mutual synchronization between a base station and a subscriber station, are transmitted over the preamble field 200. The broadcast control field 210 includes a DL (DownLink)_MAP field 211 and a UL (UpLink)_MAP field 213. The DL_MAP field 211 is a field over which a DL_MAP message is transmitted, and information elements (IEs) included in the DL_MAP message are illustrated in Table 1 below.
TABLE 1SyntaxSizeNotesDL-MAP_Message_Format( ) { Management Message Type = 2 8 bits PHY Synchronization FieldVariableSee appropriate PHY specification. DCD Count 8 bits Base Station ID48 bits Number of DL-MAP Elements n16 bits Begin PHY Specific Section {See applicable PHY section.  for(i = 1; i <= n; i++) {For each DL-MAP element 1 to n.   DL_MAP_Information_Element( )variableSee corresponding PHY specification.   if (byte boundary) {    Padding Nibble 4 bitsPadding to reach byte boundary.   }  } }}
As illustrated in Table 1, IEs of the DL_MAP message include Management Message Type indicating a type of a transmission message, PHY (PHYsical) Synchronization Field established based on modulation and demodulation schemes applied to a physical channel to acquire synchronization, DCD Count indicating a count corresponding to a change in the configuration of a downlink channel descript (DCD) message containing a downlink burst profile, Base Station ID indicating a base station identifier, and Number of DL_MAP Elements n indicating the number of elements following the Base Station ID. Although not illustrated in Table 1, the DL_MAP message includes information on ranging codes allocated to corresponding rangings described below.
In addition, the UL_MAP field 213 is a field over which a UL_MAP message is transmitted, and IEs included in the UL_MAP message are illustrated in Table 2 below.
TABLE 2SyntaxSizeUL_MAP_Message_Format( ) { Management Message Type = 3 8 bits Uplink channel ID 8 bits UCD Count 8 bits Number of UL_MAP Elements n16 bits Allocation Start Time32 bits Begin PHY Specific Section {  for(i = 1; i < n; i + n)    UL_MAP_Information_Element {Variable      Connection ID      UIUC       Offset     }  } }}
As illustrated in Table 2, IEs of the UL_MAP message include Management Message Type indicating a type of a transmission message, Uplink Channel ID indicating an uplink channel ID in use, UCD Count indicating a count corresponding to a change in the configuration of an uplink channel descript (UCD) message containing an uplink burst profile, and Number of UL_MAP Elements n indicating the number of elements following the UCD Count. The uplink channel ID is uniquely assigned by a media access control (MAC)-sublayer.
Information designating usage of an offset written in an Offset field is included in a UIUC (Uplink Interval Usage Code) field. For example, if ‘2’ is written in the UIUC field, it indicates that a starting offset used for initial ranging is written in the Offset field. Alternatively, if ‘3’ is written in the UIUC field, it indicates that a starting offset used for bandwidth request ranging or maintenance ranging is written in the Offset field. As described above, a starting offset used for initial ranging, bandwidth request ranging, or maintenance ranging, based on the information written in the UIUC field, is written in the Offset field. Information on a characteristic of a physical channel to be transmitted over the UIUC field is written in a UCD message.
If a subscriber station has failed to perform successful ranging, it sets a particular backoff value in order to increase a success rate at the next attempt, and makes a ranging attempt after a lapse of the backoff time. In this case, information necessary for setting the backoff value is also included in the UCD message. A configuration of the UCD message will be described in detail with reference to Table 3 below.
TABLE 3SyntaxSizeNotesUCD-Message_Format( ) { Management Message Type = 08 bits Uplink channel ID8 bits Configuration Change Count8 bits Mini-slot size8 bits Ranging Backoff Start8 bits Ranging Backoff End8 bits Request Backoff Start8 bits Request Backoff End8 bits TLV Encoded Information for the overall channelVariable Begin PHY Specific Section {  for(i = 1; i < n; i + n)    Uplink_Burst_DescriptorVariable  } }}
As illustrated in Table 3, IEs of the UCD message include Management Message Type indicating a type of a transmission message, Uplink Channel ID indicating an uplink channel ID in use, Configuration Change Count being counted in a base station, Mini-slot Size indicating the number of minislots of an uplink physical channel, Ranging Backoff Start indicating a backoff start point for initial ranging, i.e., indicating a size of an initial backoff window for initial ranging, Ranging Backoff End indicating a backoff end point for initial ranging, i.e., indicating a size of a final backoff window, Request Backoff Start indicating a backoff start point for contention data and requests, i.e., indicating a size of an initial backoff window, and Request Backoff End indicating a backoff end point for contention data and requests, i.e., indicating a size of a final backoff window. Here, the backoff value indicates a kind of waiting time during which a subscriber station should wait for the next ranging if it fails in the rangings described below A base station must transmit to a subscriber station the backoff value that is time information for which the subscriber station should wait for the next ranging if it fails in the current ranging. For example, if a value given by the Ranging Backoff Start and the Ranging Backoff End is set to ‘10’, the subscriber station must perform the next ranging after passing an opportunity to perform 210 (=1024) rangings by a truncated binary exponential backoff algorithm.
In addition, the TDM fields 220 and 230 correspond to time slots assigned to subscriber stations on a TDM/TDMA (Time Division Multiple Access) basis. The base station transmits broadcast information to be broadcasted to its subscriber stations over the DL_MAP field 211 of the downlink frame using a predetermined center carrier. Upon power-on, the subscriber stations monitor all frequency bandwidths previously assigned to the subscriber stations, and detect a pilot channel signal having the highest strength, i.e., the highest pilot carrier-to-interference and noise ratio (CINR). A subscriber station determines a base station that transmitted a pilot channel signal having the highest pilot CINR, as a base station to which it currently belongs, and acquires control information for controlling its uplink and downlink and information indicating an actual data transmission/reception point by analyzing a DL_MAP field 211 and a UL_MAP field 213 of a downlink frame transmitted from the base station.
FIG. 3 is a diagram schematically illustrating an uplink frame format for an OFDM/OFDMA broadband wireless access communication system, and in particular, illustrating an uplink frame format for an IEEE 802.16a/IEEE 802.16e communication system.
Before a description of FIG. 3 is given, rangings used in the IEEE 802.16a/IEEE 802.16e communication system, i.e., initial ranging, maintenance ranging (or periodic ranging), and bandwidth request ranging, will be described.
A. Initial Ranging
The initial ranging is performed after a base station request in order to acquire synchronization the base station with a subscriber station. The initial ranging is performed to set a correct time offset and control transmission power between the subscriber station and the base station. That is, the subscriber station performs the initial ranging in order to receive, upon its power-on, a DL_MAP message and a UL_MAP message/UCD message, acquire synchronization with a base station, and then control the time offset and transmission power with the base station. Because the IEEE 802.16a/IEEE 802.16e communication system employs the OFDM/OFDMA technology, the ranging procedure requires subchannels and ranging codes, and the base station assigns available ranging codes according to goals, or types, of rangings. This will be described in more detail herein below.
Ranging codes are generated by segmenting a pseudorandom noise (PN) sequence having a predetermined length of, for example, 215 bits, in a predetermined unit. Generally, two 53-bit ranging subchannels constitute one ranging channel, and ranging codes are created by segmenting a PN code over a 106-bit ranging channel. The ranging codes generated in this way can be assigned to a maximum of 48 per subscriber stations, and a minimum of 2 ranging codes per subscriber station are applied by default to rangings of the 3 goals, i.e., initial ranging, periodic ranging, and maintenance ranging. Accordingly, different ranging codes are assigned to the rangings of the 3 goals. For example, N ranging codes are allocated for initial ranging (N RCs (Ranging Codes) for initial ranging), M ranging codes are allocated for periodic ranging (M RCs for maintenance ranging), and L ranging codes are allocated for bandwidth request ranging (L RCs for BW request ranging). The allocated ranging codes are transmitted to subscriber stations through a DL_MAP message as stated above, and the subscriber stations perform their ranging procedures by using the ranging codes included in the DL_MAP message according to their goals.
B. Periodic Ranging
The periodic ranging is periodically performed in order for a subscriber station to control a channel condition to a base station after controlling a time offset and transmission power with the base station through the initial ranging. The subscriber station performs the periodic ranging by using the ranging codes allocated for periodic ranging.
C. Bandwidth Request Ranging
The bandwidth request ranging is performed when a subscriber station requests allocation of a bandwidth in order to perform actual communication with a base station, after controlling a time offset and transmission power with the base station through the initial ranging. The bandwidth request ranging can be performed using a selected one of the following three methods: Grants, Contention-based Focused bandwidth requests for Wireless MAN-OFDM, and Contention-based CDMA bandwidth requests for Wireless MAN-OFDMA. A detailed description of the three methods will now be made herein below.
(1) Grants
The Grants method requests assignment of a bandwidth when a communication system to which a subscriber station currently belongs is a single-carrier communication system. In this method, a subscriber station performs the bandwidth request ranging, using a default CID (Connection ID) rather than its own CID. If the subscriber station fails in the bandwidth request ranging, it reattempts the bandwidth request ranging after a backoff value previously determined according to the last information received from a base station and a request status of the base station, or determines to discard a received service data unit (SDU). Herein, the subscriber station has already detected the backoff value through a UCD message.
(2) Contention-Based Focused Bandwidth Requests for Wireless MAN-OFDM
The Contention-based Focused bandwidth requests for Wireless MAN-OFDM method requests assignment of a bandwidth when a communication system to which a subscriber station currently belongs is an OFDM communication system. The Contention-based Focused bandwidth requests for Wireless MAN-OFDM method is classified again into two methods. A first method performs the bandwidth request ranging by transmitting a Focused Contention Transmission message while a subscriber station uses a default CID as described in the Grants method. A second method performs the bandwidth request ranging by transmitting a broadcast CID rather than the default CID along with an OFDM Focused Contention ID. When the bandwidth request ranging is performed by transmitting the broadcast CID together with the OFDM Focused Contention ID, a base station determines a specific contention channel and a data rate for a subscriber station.
(3) Contention-Based CDMA Bandwidth Requests for Wireless MAN-OFDMA
The Contention-based CDMA bandwidth requests for Wireless MAN-OFDMA method requests allocation of a bandwidth when a communication system to which a subscriber station currently belongs is an OFDMA communication system. The Contention-based CDMA bandwidth requests for Wireless MAN-OFDMA method is classified again into two methods. A first method performs the bandwidth request ranging as described in the Grants method, and a second method performs the bandwidth request ranging by using a CDMA (Code Division Multiple Access)-based mechanism. In the second method using the CDMA-based mechanism, the communication system uses a plurality of tones comprised of OFDM symbols, i.e., uses a plurality of subchannels. Therefore, when a subscriber station performs bandwidth request ranging, a base station applies the CDMA-based mechanism to each of the subchannels. As a result, if the base station successfully receives the bandwidth request ranging, a subscriber station that performed the bandwidth request ranging through a MAC protocol data unit (PDU) allocates a frequency bandwidth. In a REQ (REQuest) Region-Focused method, if a plurality of subscriber stations attempt bandwidth request ranging through the same subchannel using the same contention code, collision possibility is increased undesirably.
Referring to FIG. 3, the downlink frame includes an Initial Maintenance Opportunities field 300 for initial ranging and maintenance ranging (or periodical ranging), a Request Contention Opportunities field 310 for bandwidth request ranging, and SS scheduled data fields 320 containing uplink data of subscriber stations. The Initial Maintenance Opportunities field 300 has a plurality of access burst periods including actual initial ranging and periodic ranging, and a collision period in case that collision occurs between the access burst periods. The Request Contention Opportunities field 310 has a plurality of bandwidth request periods including actual bandwidth request ranging, and a collision period in case that collision occurs between the bandwidth request periods. Each of the SS scheduled data fields 320 is comprised of a plurality of SS schedule data fields (SS #1 scheduled data field to SS #N scheduled data field). Subscriber station transition gaps (SS transition gap) are located between the SS scheduled data fields (SS #1 scheduled data field to SS #N scheduled data field).
FIG. 4 is a diagram schematically illustrating a procedure for performing communication through the messages illustrated in FIGS. 2 and 3 in a broadband wireless access communication system. Referring to FIG. 4, upon a power-on, a subscriber station 400 monitors all previously assigned frequency bandwidths, and detects a pilot channel signal having the highest strength, i.e., the highest pilot CINR. The subscriber station 400 determines a base station 420 that transmitted a pilot channel signal having the highest pilot CINR, as a base station to which it currently belongs, and acquires system synchronization with the base station 420 by receiving a preamble of a downlink frame transmitted from the base station 420.
If system synchronization is acquired between the subscriber station 400 and the base station 420 in this way, the base station 420 transmits a DL_MAP message and a UL_MAP message to the subscriber station 400 (Steps 411 and 413). The DL_MAP message, as described in connection with Table 1, transmits, to the subscriber station 400, information necessary for acquiring synchronization with the base station 420 by the subscriber station 400 in a downlink and information on a structure of a physical channel capable of receiving, through the synchronization, messages transmitted to the subscriber stations 400 in the downlink. The UL_MAP message, as described in connection with Table 2, transmits, to the subscriber station 400, information on a scheduling period of the subscriber station and a structure of a physical channel.
The DL_MAP message is periodically transmitted from a base station to all subscriber stations, and when a subscriber station can continuously receive the message, it is said that the subscriber station is synchronized with the base station. That is, subscriber stations that succeeded in receiving the DL_MAP message can receive all messages transmitted through a downlink.
As descried in conjunction with Table 3, when a subscriber station fails in access, a base station transmits to the subscriber station the UCD message containing information representing an available backoff value.
However, when performing the ranging, the subscriber station 400 transmits an RNG_REQ message to the base station 420 (Step 415). Upon receiving the RNG_REQ message, the base station 420 transmits to the subscriber station 400 an RNG_RSP message containing the above-stated information for correcting frequency, time, and transmission power (Step 417).
A configuration of the RNG_REQ message is illustrated in Table 4 below.
TABLE 4SyntaxSizeNotesRNG-REQ_Message_Format( ) { Management Message Type = 48 bits Downlink Channel ID8 bits Pending Until Complete8 bits TLV Encoded InformationVariableTLV specific}
In Table 4, Downlink Channel ID represents a downlink channel ID for a channel that the subscriber station received from the base station, and Pending until Complete represents priority of a transmission ranging response. For example, Pending until Complete=0 indicates that a previous ranging response has priority over other ranging responses, while Pending until Complete≠0 indicates that a currently-transmitted ranging response has priority over other ranging responses.
In addition, a configuration of the RNG_RSP message responsive to the RNG_REQ message of Table 4 is illustrated in Table 5 below.
TABLE 5SyntaxSizeNotesRNG-REQ_Message_Format( ) { Management Message Type = 58 bits Uplink Channel ID8 bits TLV Encoded InformationVariableTLV specific}
In Table 5, Uplink Channel ID represents an uplink channel ID for an RNG_REQ message that the base station received.
The OFDMA communication system proposed in an IEEE 802.16a may replace the RNG_REQ message by using a method of designating a dedicated ranging period to more efficiently perform the ranging and transmitting ranging codes for the dedicated period. A communication procedure in the OFDMA broadband wireless access communication system is illustrated in FIG. 5.
Referring to FIG. 5, a base station 520 transmits a DL_MAP message and a UL_MAP message to the subscriber station 500 (Steps 511 and 513), and details thereof are equal to those described in connection with FIG. 4. Further, as described above, in the OFDMA communication system, a ranging code is transmitted instead of the RNG_REQ message used in FIG. 4 (Step 515), and upon receiving the ranging code, the base station 520 transmits an RNG_RSP message to the subscriber station 500 (Step 517).
However, new information must be added for writing information responsive to the ranging code transmitted to the base station in the RNG_RSP message. The new information that must be added to the RNG_RSP message includes:
1. Ranging Code: received ranging CDMA code;
2. Ranging Symbol: OFDM symbol in the received ranging CDMA code;
3. Ranging subchannel: ranging subchannel in the received ranging CDMA code; and
4. Ranging frame number: frame number in the received ranging CDMA code.
As descried above, the IEEE 802.16a communication system does not take mobility of a subscriber station into consideration, i.e., it considers that the subscriber station is located in a fixed position, and considers only a single-cell configuration. However, as described above, it is provided that the IEEE 802.16e communication system considers mobility of a subscriber station in addition to characteristics of the IEEE 802.16a communication system. Therefore, the IEEE 802.16e communication system must consider mobility of a subscriber station in a multi-cell environment. In order to provide mobility of a subscriber station in the multi-cell environment, modification of operations of the subscriber station and the base station is necessary. However, the IEEE 802.16e communication system has proposed no specification for the multi-cell environment and the mobility of a subscriber station. Therefore, in order to support the mobility of a subscriber station, the IEEE 802.16e communication system requires a method for performing a handover taking an idle state and also a communication state into consideration.